Anxiety is present in high rates in both children and adults on the autism spectrum. An increasing number of studies have highlighted the potentially important role that intolerance of uncertainty may have in anxiety for those on the spectrum, as well as their interrelationships with sensory sensitivities and repetitive behaviours. In response to a lack of studies involving adults, this study examined self-report survey data regarding intolerance of uncertainty, sensory sensitivities, repetitive behaviours and anxiety in a sample of 176 adults on the autism spectrum (mean age = 42). Intolerance of uncertainty and anxiety were both found to be elevated relative to non-autistic adults (N = 116) and significant, positive correlations were found between intolerance of uncertainty, anxiety, repetitive behaviours and sensory sensitivities in those on the spectrum. Intolerance of uncertainty was found to be a significant mediator between sensory sensitivities and anxiety, as well as between anxiety and insistence on sameness behaviours. These results were not sensitive to age. Intolerance of uncertainty is an important factor to be considered in the conceptualisation and management of elevated rates of anxiety for adults on the autism spectrum.